Perl 6 code is compiled per compilation unit, or compunit for short. These are loaded with a use, need or require statement (usually at compile time). Or they are created as a string in a variable and compiled with an EVAL statement (usually at runtime). This synopsis is mostly about compunits loaded at compile time.

In the common vernacular of a Perl 5 developer, a module is the same as the file that contains the source code of a package. For Perl 6 developers, this is generally not much different. However, such a module is really a compunit that may contain 0 or more package-like statements, some of which may be module. Confusing? Yes it is. On top of that, Perl 6 allows different versions of the same compunit to be installed in a single repository. And it allows compunits from other languages to be loaded.

In Perl, the use statement is really just telling Perl to find a compunit for the given name and load its contents. Whether that name is the name of a package, module, class, grammar or role, a combination of these or something else entirely in a slang, is not known at the moment the compunit is searched for (and hopefully found). Only when the contents of the compunit are compiled, does Perl find out what's inside.

In Perl 5, the compunit's name to filename translation is generally mechanical. Foo::Bar will always refer to Foo/Bar.pm in a directory: it does not need any outside information to find out the name of the file to be loaded.

Perl 6 however, only knows about repositories that contain compunits. Each repository is represented as an object with the CompUnitRepo interface in the @?INC array. Whenever a compunit needs to be loaded, each element in the @?INC is queried for the compunit, and any candidates are returned. An exception occurs if there is no candidate, or more than one candidate that matches the requirement. Otherwise, the contents of the compunit is then obtained and loaded.

There are 2 standard implementations of the CompUnitRepo interface: CompUnitRepo::Local::File and CompUnitRepo::Local::Installation. The first is used whenever you're specifying the -I parameter when running Perl 6. The second is always used when searched for locally installed compunits as files in directories. Of course, one is free to devise any other way of storing and searching for compunits, as long as API is the same.

As in Perl 5, a module is just a kind of package. Unlike in Perl 5, modules and classes are declared with separate keywords, but they're still just packages with extra behaviors. In the case of modules, the extra behavior is the availability of the export trait and any associated support for Perl 6 standard export semantics.

A module is declared with the module keyword. There are two basic declaration syntaxes:

A named module declaration can occur as part of an expression, just like named subroutine declarations.

Since there are no barewords in Perl 6, module names must be predeclared, or use the sigil-like ::ModuleName syntax. The :: prefix does not imply globalness as it does in Perl 5. (Use GLOBAL:: for that.)

A bare (unscoped) module declarator declares a nested our module name within the current package. However, at the start of a compunit, the current package is GLOBAL, so the first such declaration in the file is automatically global.

You can use our module to explicitly declare a module in the current package. To declare a lexically scoped module, use my module. Module names are always searched for from innermost scopes to outermost. As with an initial ::, the presence of a :: within the name does not imply globalness (unlike in Perl 5).

The default package for the main program is GLOBAL. (Putting module GLOBAL; at the top of your program is redundant, except insofar as it tells Perl that the code is Perl 6 code and not Perl 5 code. But it's better to say use v6 for that.)

Every compunit has a UNIT package, which gets a lexically scoped EXPORT package automatically. Declarations marked as is export are bound into it, with their tagsets as inner packages. For example, the sub bar above will bind as UNIT::EXPORT::DEFAULT::<&bar>, UNIT::EXPORT::ALL::<&bar>, and UNIT::EXPORT::others::<&bar>.

Tagset names consisting entirely of capitals are reserved for Perl.

Exports contained within a module will also be bound into an our-scoped EXPORT package nested in the module, again with the tagsets forming inner packages. This is so the import keyword can be used with a package to import from it; the lexical IMPORT package in UNIT, on the other hand, is the only thing that is considered by use for importing.

Inner packages automatically add their export list to packages in all their outer scopes (including UNIT):

The default EXPORTALL handles symbol exports by removing recognized export items and tagsets from the argument list, then calls the EXPORT subroutine in that package (if there is one), passing in the remaining arguments.

If the exporting module is actually a class, EXPORTALL will invoke its EXPORT method with the class itself as the invocant.

In the absence of a specific scoping specified by the caller, one may also specify a different scoping default by use of :MY or :OUR tags as arguments to is export. (Of course, mixing incompatible scoping in different scopes is likely to lead to confusion.)

The use declaration is actually a composite of two other declarations, need and import. Saying

use Sense <&common @horse>;

breaks down into:

need Sense;
import Sense <&common @horse>;

These further break down into:

BEGIN {
my $target ::= OUTER;
for <Sense> {
my $scope = load_compunit(find_compunit_defining($_));
# install the name of the type
$target.install_alias($_, $scope{$_}) if $scope{$_}:exists;
# get the package declared by the name in that scope,
my $package = $_ ~ '::';
# if there isn't any, then there's just the type...
my $loaded_compunit = $scope{$package} or next;
# get a copy of the package, to avoid action-at-a-distance
# install it in the target scope
$target{$package} := $loaded_compunit.copy;
# finally give the chance for the module to install
# the selected symbols
$loaded_compunit.EXPORTALL($target, <&common @horse>);
}
}

The need declarator takes a list of modules and loads them (at compile time) without importing any symbols. It's good for loading class modules that have nothing to export (or nothing that you want to import):

need ACME::Rocket;
my $r = ACME::Rocket.new;

This declaration is equivalent to Perl 5's:

use ACME::Rocket ();

Saying

need A,B,C;

is equivalent to:

BEGIN {
my $target ::= OUTER;
for <A B C> {
my $scope = load_compunit(find_compunit_defining($_));
# install the name of the type
$target.install_alias($_, $scope{$_}) if $scope{$_}:exists;
# get the package declared by the name in that scope,
my $package = $_ ~ '::';
# if there isn't any, then there's just the type...
my $loaded_compunit = $scope{$package} or next;
# get a copy of the package, to avoid action-at-a-distance
# install it in the target scope
$target{$package} := $loaded_compunit.copy;
}
}

The importation into your lexical scope may also be a separate declaration from loading. This is primarily useful for modules declared inline, which do not automatically get imported into their surrounding scope:

(The .import_realias requires that the symbols to be imported already exist; this differs from .import_alias, which requires that the imported symbols do not already exist in the target scope.)

Additionally, the require expression evaluates to the value which is installed as the alias, so that (require Foo::Bar).new and similar expressions do the most useful thing.

Alternately, a filename may be mentioned directly, which installs a package that is effectively anonymous to the current lexical scope, and may only be accessed by whatever global names the module installs:

Only explicitly mentioned names may be so imported. In order to protect the run-time sanctity of the lexical pad, it may not be modified by require. Tagsets are assumed to be unknown at compile time, hence tagsets are not allowed in the default import list to :MY, but you can explicitly request to put names into the :OUR scope, since that is modifiable at run time:

require Sense <:ALL> # does not work
require Sense :MY<ALL> # this doesn't work either
require Sense :OUR<ALL> # but this works

If the import list is omitted, then nothing is imported. Since you may not modify the lexical pad, calling an importation routine at runtime cannot import into the lexical scope, and defaults to importation to the package scope instead:

require Sense;
Sense.EXPORTALL; # goes to the OUR scope by default, not MY

(Such a routine may rebind existing lexicals, however.)

When you pass a string, require always assumes the string contains a filename. To specify both a module name and a filename, use a colonpair modifier:

At minimum, this will create the Sense package at compile time, even if the require never puts anything into it at run time. (Sound practice would keep it consistent with whatever the require is going to do later, however.) By default the package is created as an our package unless it has already been declared my earlier.

It is also possible to specify the compunit name indirectly by string:

my $sense = "Sense";
require ::($sense) <common @horse>;

The parser will have no idea what your module is actually going to be called, so it installs no package name known at compile time. Other than that, the semantics are identical to the direct form.

Whenever an authority (such as a CPAN author or a company) posts a compilation unit as part of a distribution of Perl 6 code, or enters it into any Perl 6 library, the module is required to declare its full name so that installations can know its unique, immutable identity, such that multiple versions by different authors can coexist, all of them available to any installed version of Perl. (For the purposes of "standard Perl 6 library" we do not just mean publicly shared libraries such as CPAN, but also any internal or site-wide libraries that are shared outside the given module's dev group.)

Such modules are also required to specify exactly which version (or versions) of Perl they are expecting to run under, so that future versions of Perl can emulate older versions of Perl (or give a cogent explanation of why they cannot). This will allow the language to evolve without breaking existing widely used modules. (Perl 5 library policy is notably lacking here; it would induce massive breakage even to change Perl 5 to make strictness the default.) If an installed module breaks because it declares that it supports future versions of Perl when it doesn't, then it must be construed to be the module's fault, not Perl's. If Perl evolves in a way that does not support emulation of an older version (at least, back to 6.0.0), then it's Perl's fault (unless the change is required for security, in which case it's the fault of the insensitive clod who broke security :).

The internal API for package names is always case-sensitive, even if the library system is hosted on a system that is not case-sensitive. Likewise internal names are Unicode-aware, even if the filesystem isn't. This implies either some sort of name mangling capability or storage of intermediate products into a database of some sort for the CompUnitRepo::Local::Installation class and similar classes. In any event, the actual storage location must be encapsulated in the library system such that it is hidden from all language level naming constructs. (Provision must be made for interrogating the library system for the actual location of a module, of course, but this falls into the category of introspection.) Note also that distributions need to be distributed in a way that they can be installed on case-insensitive systems without loss of information. That's fine, but the language-level abstraction must not leak details of this mechanism without the user asking for the details to be leaked.

As discussed in Units, the required parts for library insertion are the (long) name of the compilation unit, a URI identifying the author/authority (called auth to be intentionally ambiguous), its version number (ver for short) and an optional API indicator. For example, if a compunit contains this configuration in its META6.json:

"name" : "Dog",
"version" : "1.2.1"
"api" : "woof"

and has been installed from an author identified by cpan:JRANDOM, then when you then attempt to load a compilation unit, like:

use Dog;

you're really wildcarding the unspecified bits:

use Dog:auth(Any):ver(Any):api(Any);

And when you say:

use Dog:<1.2.1>;

you're really asking for:

use Dog:auth(Any):ver<1.2.1>:api(Any);

Saying 1.2.1 specifies an exact match on that part of the version number, not a minimum match. To match more than one version, put a range operator as a selector in parens:

use Dog:ver(v1.2.1..v1.2.3);
use Dog:ver(v1.2.1..^v1.3);
use Dog:ver(v1.2.1..*);

When specifying the version of your own module, 1.2 is equivalent to 1.2.0, 1.2.0.0, and so on. However use searches for modules matching a version prefix, so the subversions are wildcarded, and in this context :ver<1.2> really means :ver<1.2.*>. If you say:

you're asking for any version of Perl 6. You need to say something like

use Perl:<6.0>;
use Perl:<6.0.0>;
use Perl:<6.2.7.1>;

if you want to lock in a particular set of semantics at some greater degree of specificity. And if some large company ever forks Perl, you can say something like:

use Perl:auth<cpan:TPF>

to guarantee that you get the unembraced Perl. :-)

When it happens that the same module is available from more than one auth, and the desired auth is not specified by the use, the version lineage that was created first wins, unless overridden by local policy or by official abandonment by the original auth (as determined either by the author or by community consensus in case the author is no longer available or widely regarded as uncooperative). An abandoned lineage will be selected only if it is the only available lineage of locally installed modules.

Once the auth is selected, then and only then is any version selection done. This implies that all installed compunits record permanently when they were first installed in the library, and this creation date is considered immutable. This date can be specified with the uppercase block typename CREATED. For example:

=CREATED 20130626 - it was a good day

As with the other credentials of the compunit, only the first \S+ are used and expected to be in the form YYYYMMDD.

When loading a compilation unit with wildcards, any valid smartmatch selector works:

In any event, however you select the compunit, its full name is automatically aliased to the short name for the rest of your lexical scope. So you can just say

my Dog $spot .= new("woof");

and it knows (even if you don't) that you meant the one by cpan:JRANDOM and version 1.3.4.

If you need to have two different versions of the same compunit loaded at the same time, you can specify the name of the compunit separately with the :name adverb:

use OldDog:name<Dog>:auth<cpan:JRANDOM>:ver<1.2.1>;
use OldDog:<Dog cpan:JRANDOM 1.2.1>; # same thing

This would alias Dog of cpan:JRANDOM, version 1.2.1 to OldDog in the current lexical scope.

The use statement also allows an external language to be specified in addition to (or instead of) an authority, so that you can use modules from other languages. The from adverb also parses any additional parts as short-form arguments. For instance:

use Whiteness:from<perl5>:name<Acme::Bleach>:auth<cpan:DCONWAY>:ver<1.12>;
use Whiteness:from<perl5 Acme::Bleach cpan:DCONWAY 1.12>; # same thing

The string form of a version recognizes the * wildcard in place of any position. It also recognizes a trailing +, so

:ver<6.2.3+>

is short for

:ver(v6.2.3 .. v6.2.*)

And saying

:ver<6.2.0+>

specifically rules out any prereleases.

If two different compunits in your program require two different versions of the same compunit, Perl will simply load both versions at the same time. For compunits that do not manage exclusive resources, the only penalty for this is memory, and the disk space in the library to hold both the old and new versions. For compunits that do manage an exclusive resource, such as a database handle, there are two approaches short of requiring the user to upgrade. The first is simply to refactor the compunit into a stable supplier of the exclusive resource that doesn't change version often, and then the outer wrappers of that resource can both be loaded and use the same supplier of the resource.

The other approach is for the compunit to keep the management of its exclusive resource, but offer to emulate older versions of the API. Then if there is a conflict over which version to use, the new one is used by both users, but each gets a view that is consistent with the version it thinks it is using. Of course, this depends crucially on how well the new version actually emulates the old version.

It's not necessary to force Perl 6 if the interpreter or command specified already implies it, such as use of a "#!/usr/bin/perl6" shebang line. Nor is it necessary to force Perl 6 in any file that begins with the unit, class, module, grammar or role keywords.

In order that language processing tools know exactly what language they are parsing, it is necessary for the tool to know exactly which variant of Perl 6 is being parsed in any given scope. All Perl 6 compilation units that are complete files start out at the top of the file in the Standard Dialect (which itself has versions that correspond to the same version of the official Perl test suite). EVAL strings, on the other hand, start out in the language variant in use at the point of the EVAL call, so that you don't suddenly lose your macro definitions inside EVAL.

All language tweaks from the start of the compilation unit must be tracked. Tweaks can be specified either directly in your code as macros and such, or such definitions may be imported from a package. As the compiler progresses through the compilation unit, other grammars may be substituted in an inner lexical scope for an outer grammar, and parsing continues under the new grammar (which may or may not be a derivative of the standard Perl grammar).

Language tweaks are considered part of the interface of any package you import. Version numbers are assumed to represent a combination of interface and patch level. We will use the term interface version to represent that part of the version number that represents the interface. For typical version number schemes, this is the first two numbers (where the third number usually represents patch level within a constant interface). Other schemes are possible though. (It is recommended that branches be reflected by differences in authority rather than differences in version, whenever that makes sense. To make it make sense more often, some hierarchical authority-naming scheme may be devised so that authorities can have temporary subauthorities to hold branches without relinquishing overall naming authority.)

So anyway, the basic rule is this: you may import language tweaks from your own private (user-library as specified in @?INC) code as you like; however, all imports of language tweaks from the installed Perl 6 library must specify the exact interface version of the package.

Such installed interface versions must be considered immutable on the language level, so that once any language-tweaking compunit is in circulation, it may be presumed to represent a fixed language change. By examination of these interface versions a language processing tool can know whether it has sufficient information to know the current language.

In the absence of that information, the tool can choose either to download and use the compunit directly, or the tool can proceed in ignorance. As an intermediate position, if the tool does not actually care about running the code, the tool need not actually have the complete compunit in question; many language tweaks could be stored in a database of interface versions, so if the tool merely knows the nature of the language tweak on the basis of the interface version it may well be able to proceed with perfect knowledge. A compunit that uses a well-behaved macro or two could be fairly easily emulated based on the version info alone.

But more realistically, in the absence of such a hypothetical database, most systems already come with a kind of database for distributions that have already been installed. So perhaps the most common case is that you have downloaded an older version of the same distribution, in which case the tool can know from the interface version whether that older distribution represents the language tweak sufficiently well that your tool can use the interface definition from that distribution without bothering to download the latest patch.

Note that most compilation units do no language tweaking, and in any case cannot perform language tweaks unless these are explicitly exported.

Compilation units that export multis are technically language tweaks on the semantic level, but as long as those new definitions modify semantics within the existing grammar (by avoiding the definition of new macros or operators), they do not fall into the language tweak category. Compilation units that export new operators or macros are always considered language tweaks. (Unexported macros or operators intended only for internal use of the module itself do not count as language tweaks.)

The requirement for immutable interfaces extends transitively to any packages imported by a language tweak module. There can be no indeterminacy in the language definition either directly or indirectly.

It must be possible for any compilation unit to be separately compiled without knowledge of the lexical or dynamic context in which it will be embedded, and this separate compilation must be able to produce a deterministic profile of the interface. It must be possible to extract out the language tweaking part of this profile for use in tools that wish to know how to parse the current language variant deterministically.